libc: enable IPTOS_MINCOST, fixup gethostbyaddr() proto.

Make netinet/ip.h have IPTOS_MINCOST which matches what
  kernel/common/linux/ip.h has to say.
Fixup gethostbyaddr() 1st arg to match what man has to say.

Change-Id: Iff9647d4a46ea88b1fc32163eb69bb9b27cdf370
diff --git a/libc/include/netinet/ip.h b/libc/include/netinet/ip.h
index 541905c..629ed77 100644
--- a/libc/include/netinet/ip.h
+++ b/libc/include/netinet/ip.h
@@ -83,7 +83,7 @@
 #define	IPTOS_LOWDELAY		0x10
 #define	IPTOS_THROUGHPUT	0x08
 #define	IPTOS_RELIABILITY	0x04
-/*	IPTOS_LOWCOST		0x02 XXX */
+#define	IPTOS_MINCOST		0x02
 #if 1
 /* ECN RFC3168 obsoletes RFC2481, and these will be deprecated soon. */
 #define IPTOS_CE		0x01	/* congestion experienced */